In the anatomy of the human heart, the left atrium receives oxygenated blood from the lungs through the pulmonary veins. The mitral valve separates the left atrium from the left ventricle. During diastole, as the contraction triggered by the sinoatrial node progresses through the atria, oxygenated blood passes through the mitral valve into the left ventricle. In this phase, the aortic valve leading into the ascending aorta closes, allowing the left ventricle to fill with blood. A similar flow of venous blood occurs from the right atrium through the tricuspid valve to the right ventricle. Once the ventricles are full, they contract during the systolic phase and pump blood out of the heart. During systole, the mitral valve closes and the aortic valve opens, thus preventing blood from regurgitating into the left atrium and forcing blood into the aorta, and from there throughout the body. Because of the high pressures associated with the left ventricle during systole, proper functioning of the mitral valve to prevent blood from flowing back through the system is extremely important.
The various anatomical components of a healthy mitral valve are depicted in FIG. 1. The mitral annulus MA comprises a fibrous ring encircling the orifice between the left atrium LA and the left ventricle LV. The average cross-sectional area of the human mitral annulus is 4-10 cm2. The mitral valve is a bicuspid valve having a posterior leaflet PL and an anterior leaflet AL. Chordae tendenae CT (or simply “chordae”) extend from the free edges and the bases of the two leaflets to a pair of papillary muscles located in the LV. The two papillary muscles are located along the anteriolateral and the posteromedial wall of the LV and are therefore referred to as the anteriolateral papillary muscle AP and the posteromedial papillary muscle PP, respectively.
Normal dilatation of the left ventricle and downward displacement of the papillary muscles AP and PP pulls the chordae tendoneae CT, which in turn pull the leaflets open. When the ventricles contract the papillary muscles are displaced upward, and the distance h between the papillary muscles and the annulus is reduced. The chordae tendonae become slack, allowing the leaflets to come together or “coapt.” As seen in FIG. 1, the leaflets coapt along a substantial surface area in the normal functioning heart, with the free edges of the leaflets mutually bending toward the left ventricle LV. For purpose of discussion, the mitral annulus MA of a normal, healthy heart lies generally in a datum plane 20 defined perpendicular to the average blood flow direction 22 through the mitral valve MV. Although a typical mitral annulus MA may be three-dimensional, the datum plane 20 is representative of the relative positions of the anterior and posterior side of the annulus.
In patients who suffer from a heart attack or cardiomyopathy, regions of the left ventricle lose their contractility and dilate. Dilation of the left ventricle is often associated with a down and outward displacement of the papillary muscles. The change in the location of the papillary muscles increases the distance between the papillary muscles and the mitral valve leaflets. Since the chordae tendenae do not change their length significantly, the chordae tend to pull or “tether” the leaflets. In severe cases of left ventricle dilatation, the tethering of the chordae prevents the leaflets from coapting resulting in mitral regurgitation. Since this type of regurgitation is not associated with any disease or damage of the mitral apparatus it is often referred to as “functional” mitral regurgitiation. Dilation of the left ventricle LV is also a symptom associated with mitral regurgitation in patients with iopathic dilated cardiomyopathy or ischemic cardiomyopathy, and in patients with long-standing valvular regurgitation from other etiologies such as myxomatous disease, endocarditis, congenital defects, or rheumatic valvular disease.
As seen in FIG. 2, dilation of the left ventricle LV generally increases the distance h′ between the papillary muscles PM1 and PM2 and the mitral annulus MA. The increased distance h′ between the papillary muscles PM1 and PM2 and the mitral annulus MA in turn increases the tension in the chordae tendonae CT and may create a depression of the posterior aspect of the annulus below the datum plane 20, but this depression is not pronounced enough to reduce h′. The resulting increased tension in the chordae reduces the ability of the leaflets to come together during systole, which can lead to mitral valve insufficiency.
FIGS. 3a-3c illustrate the normal and abnormal mitral valve from the left atrium as exposed during surgery, that is, in atrial plan view. The anterior aspect of the mitral annulus MA forms a part of the “cardiac skeleton” and includes left and right fibrous trigones, LT and RT. The left trigone LT and right trigone RT are indicated at the junction points of the anterior leaflet AL and posterior leaflet PL. These junction points are also known as anteriolateral and posteriomedial trigones or commissures between the leaflets. The posterior aspect of the mitral annulus MA, in contrast to the anterior aspect, consists mainly of muscular tissue of the outer wall of the heart. The posterior leaflet PL is divided into three scallops indicated as P1, P2, and P3 in sequence from the left trigone LT counterclockwise to the right trigone RT. FIG. 3a shows the mitral valve and the papillary muscles of a normal mitral valve viewed from the left atrium. FIG. 3b illustrates the effect of an infarct of the posteromedial wall of the left ventricle LV on the geometry of the mitral apparatus, tending to cause an asymmetric dilation. The infarct causes the posteriomedial wall to dilate moving the posteromedial papillary muscle PP outward. The chordae tendenae CT connected to the posteromedial papillary muscle PP pull the free margins of the posterior leaflet PL and anterior leaflet AL away from the natural line of coaptation. A gap is created between the leaflets along the P2-P3 region of the posterior leaflet PL. Asymmetric dilatation of the left ventricle LV associated with a regurgitating jet in the P2-P3 region is most common in patients with ischemic mitral regurgitation. In contrast, FIG. 3c illustrates functional mitral regurgitation in the case of symmetrical dilatation of the LV. Both papillary muscles AP and PP move outward, stretching and pulling the whole posterior leaflet PL outward. Symmetrical dilatation of the LV is associated with dilatation of the posterior segment of the mitral annulus and a central regurgitant jet along the P2 region of the PL. Symmetrical dilatation of the LV is most common in patients with cardiomyopathy.
Mitral valve insufficiency is commonly treated by repairing or replacing the mitral valve. The most widely accepted technique for mitral valve repair is the remodeling of the mitral annulus with an annuloplasty. The goal of the annuloplasty is two-fold: reduction of the annular area to its normal size and reshaping of the annulus to re-establish the normal geometry of a health mitral annulus. In case of functional mitral regurgitation, the root cause of the insufficiency is the dilation of the LV and the associated dislocation of the papillary muscle. The purpose of the annuloplasty is to compensate for the dilation of the LV by reducing the cross-sectional area beyond its natural size. The downsized annulus brings the two leaflets closer together re-establishing coaptation of the leaflets.
Prostheses for annuloplastic surgery available on the market are generally of two types, with some hybrids. Flexible annular prostheses, made of various materials, that allow a “linear” reduction of the annular circumference, and rigid and semi-rigid annular prostheses made of various materials, that allow the “linear” reduction of the annular circumference and a geometric remodelling so as to re-establish the physiological systolic shape of the annulus. Additionally, semi-rigid prostheses permit some deformation in order to allow the prosthesis to follow the deformations of the annulus—during the cardiac stages. All the rigid and semi-rigid annular prostheses have a kidney-like or coupled D shape, with an anterior half-ring, rectilinear in first approximation, that gets sutured in correspondence with the anterior valve leaflet and a curved posterior half-ring that is sutured in correspondence with the posterior valve leaflet. The shape of the annular prostheses at issue reproduces the configuration of the valve annulus during the ventricular systole, and therefore in the stage of the valve closing. The ratio between minor axis and major axis is approximately 3:4 in all the models currently on the market since it reproduces normal anatomical ratios.
The “downsizing” technique involves, for example, selecting a 26 mm ring for a nominal 28 mm annulus, while still maintaining the minor axis/major axis size ratio of approximately 3:4. The size nomenclature refers to the width of the major axis. Although good results have been reported with the downsizing technique, the reliability and durability of this operation to correct ischemic mitral valve insufficiency are not as good as for other causes of mitral valve insufficiency using the same techniques. This is largely due to the fact that a remodeling annuloplasty with currently available rings corrects only one anomaly, and various other functional anomalies seen in ischemic mitral valve insufficiency may not be corrected as effectively.
Annuloplasty rings have been developed in various shapes and configurations over the years to correct mitral regurgitation and other conditions that reduce the functioning of the valve. For example, Carpentier, et al. in U.S. Pat. No. 4,055,861 disclosed two semi-rigid supports for heart valves, one of which being closed (or D-shaped) and the other being open (or C-shaped). In the closed configuration, the ring is generally symmetric about an anterior-posterior plane, and has a convex posterior side and a generally straight anterior side. U.S. Pat. Nos. 5,104,407, 5,201,880, and 5,607,471 all disclose closed annuloplasty rings that are bowed slightly upward on their anterior side. Because the anterior aspect of the mitral annulus MA is fibrous and thus relatively inflexible (at least in comparison to the posterior aspect), the upward curve in the anterior side of each ring conforms that ring more closely to the anatomical contour of the mitral annulus. This three dimensional configuration reduces undue deformation of the annulus.
In general, conventional annuloplasty rings are intended to restore the original configuration of the mitral annulus MA. When correcting a condition as seen in FIG. 2, high stresses are created in the sutures connecting the annuloplasty ring to posterior aspect of the annulus because the “overcorrecting ring,” i.e., a ring 1 to 2 sizes smaller than the normal size “pulls” the annulus inward and upward. The stresses sometimes result in the dehiscence or separation of the ring from the annulus because the sutures pull through the tissue.
It should be noted here that correction of the aortic annulus requires a much different ring then with a mitral annulus. For example, U.S. Pat. Nos. 5,258,021 and 6,231,602 disclose sinusoidal or so-called “scalloped” annuloplasty rings that follow the up-and-down shape of the three cusp aortic annulus. Such rings would not be suitable for correcting a mitral valve deficiency.
While good results in the treatment of mitral valve insufficiency, congestive heart failure, and mitral regurgitation have been obtained in the preliminary applications of the above-described methods and apparatuses, it is believed that these results can be significantly improved. Specifically, it would be desirable to produce a mitral annuloplasty ring that takes into consideration all of the dysfunctions that exist in ischemic mitral valve insufficiency, namely, the dilatation of the annulus, the asymmetrical deformation of the annulus, and the increased distance between the posterior papillary muscle and the annulus.